After doing the math, it's looking like my rough estimate was a little high, but I suspect the numbers I'm about to share is higher than you're thinking.
Sure, they qualify for Medicare, but that doesn't mean they won't be still be spending serious money on healthcare. According to CNBC, if you were retiring in 2019, as a couple in good health you'll need on average of $390,000[1]. That assumes you're on Medicare as well. I'm guessing most people here are about 35 years old or younger, so not eligible for Medicare for at least another 30 years.
If I assume 2.5% inflation over the next 30 years (the default given in the following link[2]), that $390k in 2019 becomes $860k in 2051.
Granted that seems to be the average, and there will be people with less expenses. But that's just healthcare. While you'll probably own your home outright by then, there's the eventual assisted living that might be in your future as well, which already can cost up to $1500-4000 a month today (with monthly expenses another $2k+)[3], let alone 40-50 years from now. Although I suspect some people help pay for that by selling their homes at that point.
So okay, maybe you don't need to be a multi-millionaire when you retire to be comfortable, but you probably should try to be a millionaire, at least you and your spouse together (assuming you're going to have one).
One thing I didn't take into account was Social Security, which assuming it's still around in 2050 that will offset some of this. I do have a bad habit of assuming Social Security is going to be either severely nerfed or fall apart by the time I retire, so I tend not to account for it in my retirement planning. Maybe you could knock off $300k thanks to Social Security checks, I don't know and don't have time to dig further.
[1] https://www.cnbc.com/2019/07/18/retiring-this-year-how-much-...
[2] https://smartasset.com/investing/inflation-calculator
[3] https://www.assistedliving.org/the-average-cost-of-senior-li...